One of the most important phenomenae undermining the performance of membrane processes used to purify potable water and industrial wastewater is membrane fouling and scale formation which reduces the membrane permeate flowrate and, if not mitigated, shortens the useful life of the membrane. The deposition of fouling material or scale compounds on the surface and/or inside the pores of the membrane will take place as a result of the increase in the concentration of these compounds at the membrane surface since they are rejected (i.e. separated) by the membrane.
Typical xe2x80x9cmembrane foulantsxe2x80x9d include colloidal suspended solids such as clays and silt, metal hydroxides such as iron hydroxide originating from corrosion of steel piping and tanks, naturally occurring organic matter (NOM) including humic substances, soluble organic compounds and insoluble xe2x80x9coil and greasexe2x80x9d which are typically present in industrial effluents and xe2x80x9cbio-foulantsxe2x80x9d. The xe2x80x9cbio-foulantsxe2x80x9d can be xe2x80x9caerobicxe2x80x9d or xe2x80x9canaerobicxe2x80x9d bio-mass which form due to the bio-degradation of organic compounds in the water in the presence, or absence, of oxygen, respectively. While xe2x80x9cmembrane foulantsxe2x80x9d affect all types of membranes, including reverse osmosis membranes (RO), nano-filtration membranes (NF), ultra-filtration membranes (UF) and micro-filtration membranes (MF), scale formation typically occurs in RO and NF membranes only as a consequence of xe2x80x9cconcentrationxe2x80x9d of xe2x80x9csparingly or partially solublexe2x80x9d inorganic scale compounds including calcium carbonate, silica and calcium sulphate when these compounds are rejected by these xe2x80x9ctightxe2x80x9d membranes. Before membranes can be used to purify water from various sources, the water must be pre-treated and conditioned in order to separate suspended solids, colloids, oil and grease and NOM""s and provide a feedwater that is free from these membrane fouling compounds and is chemically compatible with the membranes.
A large number of pretreatment processes have been reported in the membrane and patent literature. These processes include the separation of colloidal suspended solids by coagulation and flocculation using inorganic multivalent metal hydroxides (e.g. aluminum sulphate or ferric chloride) and/or the highly effective flocculating cationic polymers (e.g. high molecular weight quaternary ammonium compound), respectively. This treatment is typically followed by clarification, depending on the suspended solids loading, and/or filtration using sand filters, dual-media filters or multi-media filters followed by a 5-micron cartridge filter. The multi-media filters contain a support layer of garnet or fine gravel, one or two layers of fine sand and anthracite.
Depending on the application, micro-filtration membranes may be used as an ultimate polishing step in addition to these chemical conditioning and filtration steps, or it may be used exclusively in lieu of these filtration steps. When the water contains high hardness due to calcium or magnesium, water softening resins may be used in the pre-treatment train. Activated carbon media may also be used to pre-treat industrial effluents containing organic compounds, as well as to remove free chlorine which chemically attacks the polyamide membrane film typically used in NF and RO membranes.
For example, Ebara et. al. Disclose in U.S. Pat. No. 4,080,289 a process for the treatment of industrial effluents using RO membranes in which said water is pre-treated by the addition of aluminum and/or iron salts in order to solubilize fluoride ions and prevent formation of calcium fluoride scale on the RO membranes as this compound is concentrated in the RO retentate. This patent also discloses a mechanical cleaning procedure of tubular RO membranes having surface deposits of scale and/or fouling material by using and recycling xe2x80x9csponge ballsxe2x80x9d to physically remove said scale and fouling material.
Smith discloses in U.S. Pat. No. 5,174,901 a membrane process for purifying wastewater which includes pre-treatment using a sand filter to remove particulate matter, an activated carbon filter to remove free chlorine and tri-halomethanes and a water softener to remove calcium, magnesium, iron and manganese compounds which would otherwise form scale on the membrane surface. The pre-treated water is then purified further with RO and ion exchange to separate soluble ions, organic and biological compounds and finally calcium salt and carbonic acid are added to the treated water to give a better tasting calcium-enriched water.
In another example of Prior Art, Arnaud discloses in U.S. Pat. No. 5,647,977 a system for purifying industrial effluents from laundry and vehicle washing operations. The system includes a coarse solids filtration device, aeration to fluidize and separate oil and other organic compounds, flocculation of colloidal solids, filtration of flocculated solids, activated carbon or polymeric resin to separate chlorine and organic compounds, high purity granular copper and zinc beds to separate heavy metals, two-stage anion exchange resins and two-stage cation exchange resins to remove organic compounds, prevent bio-growth and demineralize the water.
Furthermore, Boyce et. al. disclose in U.S. Pat. No. 5,651,894 a double-pass reverse osmosis process for producing ultrapure water in which the water is pre-treated by the addition of dithiocarbamate which produces a reducing environment to prevent bacterial growth and scavenge metals. The water is then purified with the 2-pass RO membranes and the RO-2 concentrate is recycled to the RO-1 feed at a reduced pressure to prevent generation of oxidants.
In U.S. Pat. No. 4,414,113, La Terra discloses a method for pre-treatment of water to be purified by hollow fiber reverse osmosis membranes. The raw water is directed into a pressure vessel containing a number of filter elements, which in turn have hollow fiber RO membranes wound around center cores such that the water flows from the outside of the filter elements towards the center cores. The pure permeate passes into the center bores of the fibers while the concentrate passes into the center cores of the elements.
Pohl et. al. disclose an RO pre-treatment method in U.S. Pat. No. 4,261,833 in which they teach precipitation and flocculation of metal hydroxides by using a combination of the acid salt and the acid having the same anion as the salt in order to produce the metal hydroxide at optimum pH. The resulting concentrated hydroxide flakes and flocculated colloids are separated in a 2-stage centrifuge, followed by acidification of the hydroxide precipitate and recycle of the acid/salt pair for further water treatment.
Henz et. al. disclose in U.S. Pat. No. 4,758,347 a process for purifying dyeing wastewaters by adjusting the pH with alkali or acid to the range 4 to 9 followed by coarse filtration to remove the suspended solids and adjusting the pH to the range 40-60xc2x0 C. The pre-treated water is then subjected to a 2-stage RO membrane system in which the second stage permeate is recycled to the pre-treatment step and the first stage permeate is sent to a wastewater treatment plant for final treatment before discharge. The second stage concentrate is sent to a wet-air oxidation or combustion system to destroy the organic compounds.
In another related prior art, Comstock et. al. disclose in U.S. Pat. No. 5,374,357 a process for removing colloidal matter from raw water by passing the water through a finely divided filter media which is impregnated with a suitable coagulant. The water-borne colloidal matter is captured and deposited on the large surface of the filter media and is subsequently removed by back-washing of the filter media.
One of the most common problems encountered in the prior art above when using inorganic coagulants and cationic (polymeric) flocculating chemicals for the pre-treatment of water before subjecting the water to membrane purification is the relatively strong positive charge of these coagulating and/or flocculating compounds. This charge is required in order to neutralize the negative surface charge of the colloidal particles, also known as the xe2x80x9cZeta Potentialxe2x80x9d, thus allowing these particles to agglomerate, flocculate, settle in the clarifier and/or separate efficiently by filtration through the media filter and cartridge filter. Unfortunately, these compounds must be applied in some excess concentrations and a residual concentration, must remain in the water after treatment in order to ensure adequate flocculation. This residual cationic flocculant concentration is deleterious to most commonly used polymeric MF, UF, NF and RO membranes.
Since most membranes possess a negative surface charge, any excess cationic polymer will be attracted to the membrane surface where it will form a chemical bond and deposit as a resilient film which is very difficult to remove by cleaning. These compounds will xe2x80x9cblindxe2x80x9d the surface and result in a sharp decline in the membrane permeate flux. Prior art does not disclose the membrane xe2x80x9cfoulingxe2x80x9d effect due to the excess cationic charge in the commonly used coagulants and flocculants.
Fouling of the membrane surface also occurs due to reaction of excess cationic polymer or multivalent cations with the negatively charged scale inhibitor compounds which are typically introduced into the water before the NF or RO membranes in order to prevent scale formation due to the water hardness compounds. Chemical reaction between the flocculating polymer and the polymeric scale inhibitor will form a xe2x80x9cstickyxe2x80x9d, resilient, high molecular weight organic substance that coats the membrane surface and results in a severe loss of the membrane permeate flux. This substance is difficult to clean and could indeed shorten the membrane""s useful life.
A number of devices and techniques have been developed in order to control and optimize the flocculant dosage in order to reduce the cost of water treatment chemicals. Most of these techniques are based on continuous measurement of the particles"" surface charge and automatic adjustment of the flocculant dosing pump output in order to control its concentration in solution. However, these techniques are only partially effective since, apart from control xe2x80x9cdifficultiesxe2x80x9d, the formation of the deleterious xe2x80x9ccationic filmxe2x80x9d on the membrane surface requires only a small concentration of the polymer, even as low as 0.1 mg/liter.
Other contaminants not adequately addressed by prior art pre-treatment processes include iron and manganese, oil and grease and soluble organic compounds. Iron hydroxides are particularly problematic for membranes since, in their chemically reduced state, they form a xe2x80x9cgelatinousxe2x80x9d film which strongly adheres to the membrane surface and also deposits and oxidizes within the membrane pores. A typical treatment method for iron (and manganese) involves the oxidation of the relatively soluble ferrous ions, Fe2+, to the insoluble ferric ions, Fe3+ using air and/or chemical oxidants, including chlorine, hydrogen peroxide and xe2x80x9cGreen Sandxe2x80x9d beds. Unfortunately, these oxidizing chemicals are xe2x80x9cnon-specificxe2x80x9d, attacking all other oxidizable organic compounds, and they are quite costly to use, especially when the concentrations of the iron and/or manganese ions are considerable, e.g.  greater than 10 mg/liter. Furthermore, the presence of excess chemical oxidant such as chlorine or hydrogen peroxide in the pre-treated water is deleterious to the polymeric membranes. On the other hand, sparging air through the contaminated water inside a storage tank and/or feed tank is not always effective in oxidizing the soluble ferrous iron compounds because of the relatively poor contacting efficiency of these devices and also because iron often occurs in its chelated form or as a relatively stable ion complex.
The presence of insoluble xe2x80x9coil and grease, OandGxe2x80x9d in the water also presents a serious technical challenge for RO and NF membranes since these membranes can not tolerate OandG concentrations in excess of 1 mg/L. Oil and grease may be removed by various techniques including the use of an oil coalescer, oil skimmer and granular activated carbon. While ultra-filtration membranes can also separate emulsified oils effectively, these membranes are quite costly to use as means to pre-treat water before RO or NF membranes. Furthermore, the presence of OandG in the wastewater is usually associated with biological growth and concomitant bio-fouling of the membranes.
On the other hand, soluble organic compounds in the water will not directly foul membranes unless their solubility limits are exceeded and they begin to form a separate phase. This may happen as the concentration of these compounds increases because of rejection (i.e. separation) by the RO or NF membranes. More importantly, however, the presence of soluble organic compounds in water is invariably associated with biological activity and the formation of a significant microbial population under aerobic, anoxic and/or anaerobic conditions, depending on the availability of oxygen. This phenomenon will result in xe2x80x9cbio-foulingxe2x80x9d of the membrane since the micro-organisms will be effectively separated by MF, UF, NF and RO membranes, and they attach themselves to the surface of the membrane where they will metabolize the organic xe2x80x9cfoodxe2x80x9d source in the water. While membrane bio-fouling represents a serious problem that must be addressed, anaerobic biological activity is particularly deleterious to the membranes, resulting in complete and often irreversible loss of membrane permeate flux.
When the concentration of biodegradable organic compounds in the water is significant, (i.e. COD  greater than 100 mg/L) these compounds are best aerobically biodegraded under controlled conditions and the sludge thus produced be separated by the appropriate clarification and/or membrane filtration before the water can be further purified by NF or RO membranes. Chemical oxidation of soluble organic compounds in water is typically practiced only when the concentration of organic compounds is low and when the nature of these compounds is such that they can be oxidized effectively and completely, producing harmless byproducts.
The present invention provides effective low-cost means for pre-treatment of raw water and industrial effluents containing these compounds, while ensuring the chemical compatibility of the pre-treated water with the membranes used downstream.
The problem of contamination of surface water from lakes and rivers with E-Coli, Giadia and Crypto-sporidium pathogens from untreated sewage and farm animal rejects has become a serious health challenge with many townships periodically imposing xe2x80x9cboil waterxe2x80x9d advisories or installing special high-cost hollow fiber or tubular micro-filtration or ultra-filtration membrane systems. Since some of these pathogens can not be destroyed by chlorination and must be removed by physical separation, membrane filtration provides the most effective, safe and reliable approach to produce potable water from contaminated surface water or ground water.
When the potable water sources have high Total Dissolved Solids (TDS), high hardness and/or colour problems, xe2x80x9ctighterxe2x80x9d membranes namely NF or RO membranes must be used. However, these latter membranes are typically fabricated from spirally-wound membrane flat sheets having different polymer chemistry than the MF and UF membranes, and are thus more prone to fouling with colloidal solids and biological solids. This will necessitate using hollow fiber or tubular MF or UF membranes upstream of the NF or RO membranes as a pre-treatment step in order to separate colloidal matter and ensure minimum fouling and adequate life of these spirally-wound membranes. These treatment procedures (i.e. using hollow fiber or tubular MF or UF in addition to spirally-wound NF or RO membranes) are costly, especially when it is required to treat large volumes of potable water.
In addition to the colloidal suspended solids associated with surface water, industrial effluents will also contain soluble iron, soluble organics and oil and grease contaminants which must also be treated and removed before the water can be purified with spirally-wound NF or RO membranes. If the water is not xe2x80x9cconditionedxe2x80x9d and pre-treated effectively, the iron hydroxides and the organic and bio-foulants will not only foul the NF or RO membranes, but they will also undermine the performance of the costly MF or UF xe2x80x9cpre-treatmentxe2x80x9d membranes.
Since the capital and operating costs of spirally-wound membranes per unit of membrane surface area are much lower than hollow fiber or tubular membranes, there is a strong economic incentive to improve the xe2x80x9cfeed waterxe2x80x9d quality by separating suspended solids, metal hydroxides, oil and grease and bio-foulants using new, effective, low-cost pre-treatment methods. With these low-cost pre-treatment methods, it would be possible to produce high quality potable water or recycled industrial water by using the relatively low-cost spirally-wound MF or UF membranes on their own when the water contains low TDS, hardness and colour, or by using spirally-wound NF or RO membranes when the water contains significant concentrations of TDS, hardness or colour, in addition to the colloidal solids and microbial contaminants.
To summarize, the problems associated with prior art, namely excessive suspended solids, residual cationic flocculant concentration, residual unoxidized iron and manganese ions, oil and grease and organic fouling and anaerobic, anoxic and aerobic biofouling, render pre-treatment ineffective or only partially effective and the performance of the ultimate membrane purification step is therefore undermined. This invention effectively addresses these limitations by applying effective, low-cost pre-treatment steps, followed by using a low-cost spirally-wound membrane purification step.
The current invention addresses the serious limitations associated with prior art pre-treatment methods and provides an effective low-cost process to treat surface water, ground water and industrial effluents in order to separate undesirable contaminants and produce xe2x80x9cpotable waterxe2x80x9d and recycled (or dischargable) industrial waste water, respectively.
This invention utilizes a xe2x80x9chigh efficacyxe2x80x9d polymeric cationic coagulant and flocculant, as per prior art, which is mixed rapidly, injected into the water and then allowed a sufficient retention time in order to bring about the efficient agglomeration of colloidal clay, iron hydroxide and NOM particles. The large floc particles thus produced can subsequently be separated by direct filtration through a suitable multi-media filter (MMF) consisting of a shallow but tough support layer of garnet, a main thick layer (i.e. approx. 1-2 ft depth) of fine sand consisting of particles in the size range 0.3-0.5 mm, and a top layer of anthracite or granular activated carbon of approximately 1 ft depth. The linear flow velocity through the MMF must be controlled at approximately 3 U.S. gpm per square foot of bed cross-sectional area in order to ensure efficient contacting and mass transfer.
One of the main features of the process of this invention is the xe2x80x9ccharge neutralizationxe2x80x9d and reversal step in which any positively charged excess or residual cationic polymer molecules or particles passing through the MMF are safely neutralized or more preferably turned into an excess negative charge which is compatible with any MF, UF, NF and RO membranes used downstream. The source and type of the negative charge applied to the water after the MMF will depend on the volume of the water being treated and the end-use of this water.
The charge neutralization step will therefore consist of a high voltage capacitor installed in the form of a probe inside the water line, at a suitable distance upstream of the final membrane treatment step. Another method of neutralizing the positive charge due to the cationic polymer involves using a suitable ion exchange resin including a cationic resin and/or a chelating resin which will adsorb the flocculant""s cationic charge preferentially, leaving chemically compatible soluble ions in the water. Another form of charge neutralization which may be introduced in the case of low volumes of treated water is achieved by the addition of an alkali solution, e.g. sodium hydroxide. Yet another method of charge neutralization is achieved by the addition of a small concentration of a polymeric anionic surfactant to the water downstream from the MMF, thus neutralizing the excess positive charge of the cationic flocculant.
The water leaving the charge neutralization/charge reversal step is filtered using a 5-micron cartridge filter to remove any insoluble compounds formed as a result of the charge neutralization step. The water thus produced will have a compatible negative charge and a low Silt Density Index (SDI) of  less than 5. This water represents a suitable feed to the spirally-wound MF, UF, NF and RO membranes which are used in the last water treatment step. Depending on the objective of the last membrane water purification step and the composition of the pre-treated raw water, either spirally-wound MF, UF, NF or RO membrane will be used. In this water purification step, the membrane will separate any remaining pathogens and biological contaminants, oil and grease, water hardness and dissolved solids, thereby producing pure permeate which is safe to drink, recycle or discharge. This permeate may be subjected to a final xe2x80x9cchlorinationxe2x80x9d treatment step in order to guard against further microbial contamination during water storage and distribution. The concentrate from the membrane treatment step is discharged, treated further or disposed of in a safe manner. Because of the above pretreated water chemical (i.e. charge) and physical (i.e. low TSS and SDI) compatibility with the membranes, it will be possible to use spirally-wound membranes in lieu of the more costly hollow fiber or tubular membranes which are sometimes used in the prior art, thus rendering this invention more economical than the prior art.
This and other embodiments of the present invention are further described in the next sections.
The present invention provides an efficient and economical method for the pre-treatment of water containing colloidal suspended solids, metal hydroxides, oil and grease, soluble organic compounds and biological solids, upstream of (i.e. before) the membrane treatment step. Effective pre-treatment of the water is needed in order to minimize the fouling potential of the membranes, maintain high membrane permeate throughput and prolong the membrane life. Four important examples illustrating the process of the invention are provided schematically in FIGS. 1, 2, 3 and 4.